Heat from a combustion reaction can be converted to a more uniform radiation, which can be in the infrared (“IR”) range, by an emitter plate which can be heated by the reaction and may become red hot. A gas/air mixture entering a distribution chamber can be dispensed uniformly over a radiating surface of the emitter, where the surface may be approximately parallel to a paper or textile web. Heat produced in a combustion chamber can be initially contained in the flue gases, and may then be converted to IR radiation.
Surfaces can emit thermal radiation when heated. However, at a particular temperature and wavelength, there can be a theoretical maximum amount of radiation that any surface can emit. A surface that emits this maximum amount of radiation can be referred to as a “black body.” Certain equations such as, e.g., Planck's Law, may be used to calculate the amount of radiation emitted by a surface as a function of wavelength and temperature. Most surfaces are not black body emitters, and are capable of emitting only a fraction of the amount of thermal radiation that a black body would emit under similar conditions. This fraction can be referred to as emissivity. For example, a surface which emits half as much radiation at a given wavelength and temperature as a black body may have an emissivity of 0.5. A “universal” black body can be considered to have an emissivity of 1.0 at all temperatures and wavelengths. Table 1 provides typical emissivities of various materials and surfaces coated with vapor deposits.
A black body surface condition (e.g., an emissivity equal to 1) can be approached by providing a deep hole (e.g., a pinhole) where radiation may be reflected several times along the walls of the hole before emanating from the hole. This type of an efficient emitter can thus be a deep object. The geometric shape of such a black body can be based on, e.g., performance, total size and/or production costs. For example, a black body can have a form of a hollow spherical shell which includes a hole in the shell, and a viewing port may be provided off-axis so that the interior surface is seen at an angle. The effective emissivity of such a spherical shell can depend on, e.g., temperature uniformity, emissivity of an interior surface of the shell, and a ratio of the viewing port size to the sphere size. It may be difficult to uniformly heat such a sphere, and this type of black body may be of primarily theoretical interest rather than having practical uses.
Other geometrical configurations may be used to construct objects that can behave similar to black bodies such as, for example, cylindrical cavities, conical cavities and/or double cones. These configurations can promote multiple reflections of emitted energy in a manner similar to that of a spherical shell. The effective emissivity of such configurations can approach a theoretical maximum (e.g., an emissivity very close to 1) if they are properly designed and fabricated. Plates that are approximately flat may generally have a lower emissivity than other shapes, but they can be preferred in practical applications where large, uniform areas are desired for emission of radiation. Such large emitting areas can be used, e.g., for drying paper webs or textiles during production thereof. Surface emissivity can be an important factor for energy radiation performance when a flat plate is used. A higher emissivity value for a surface can provide, e.g., more efficient radiation and/or cooler operating temperatures for a given rate of IR emission.
Properties and performance of IR emitters may be understood in part by considering, e.g., the Stefan-Boltzmann equation which relates a power density q radiated by a “gray body” (e.g., a radiating object having an emissivity less than that of a black body) to a temperature of the body. The Stefan-Boltzmann equation can be written as:q=Aσ∈T4  (1)where the power density q can be expressed in units of W/m2, the Stefan-Boltzmann constant σ=5.67×10-8 W/(m2-K4), a material-dependent emissivity ∈ can be between 0 and 1, inclusive, and T can represent the absolute temperature of the body in degrees Kelvin. The parameter A can represent a nominal area of the body (e.g., a cross-sectional area taken normal to a portion of the body). A surface can have an effective area which may be larger than the nominal area A. For example, a rough surface can have an effective area which may be larger than that of a similarly-sized smooth surface. A surface area density can refer to a ratio of the effective area of a body or surface to the nominal area. For example, a smooth surface can have a surface area density of approximately 1, and a rough surface may have a surface area density greater than 1. Such increases in surface area density can lead to an increase in the emissivity E as described in Eq. (1), which can be based on the nominal area A. However, surface roughening using conventional techniques such as, e.g., machining, grinding and/or peening may not provide a significant increase in the effective surface area, and the increase in surface area density provided by such techniques may diminish at high temperatures.
TABLE 1Approximate emissivity values for various materials and coatings.MaterialEmissivityBrilliant Aluminum Paint0.31Finch Aluminum Paint 643-1-10.23Dupont Silver Paint 48170.49Chromeric Silver Paint 5860.30Black Chrome0.62Black Copper0.63Black Nickel0.66Polished Nickel0.072Vapor deposited Aluminum0.02Vapor deposited Aluminum on Fiberglass0.07Vapor deposited Aluminum on Stainless Steel0.02Vapor deposited Chromium0.17Vapor deposited Germanium0.09Vapor deposited Gold0.02Iron Oxide0.56Vapor deposited Molybdenum0.21Vapor deposited Nickel0.04Vapor deposited Rhodium0.03Vapor deposited Sliver0.02Vapor deposited Titanium0.12Vapor deposited Tungsten0.27
Radiation output of an emitter can be proportional to the fourth power of the absolute temperature T, as described in Eq. (1). Minor increases in temperature can thus result in a much greater radiative output q and a corresponding increased drying effect. However, conventional emitter materials may not be suitable for constant operation at very high temperatures, which can reduce their service life (e.g., by high temperature corrosion). A maximum radiation output which can maintain a sufficiently long service life may thus be limited by the emitter material's resistance to a degradation at high temperatures.
Energy from the combustion reaction, which may be contained in a flue gas, can be transferred convectively to an emitter, e.g., by flow of the hot gas along a solid body. This mechanism can affect the efficiency of gas-fired IR emitters. For example, energy emitted in the form of IR radiation may first be transferred convectively to the emitter. Thus, both thermal radiation and convective heat transfer can affect the performance of gas-fired infrared emitters.
A specific convective heat transfer rate Q, in units of W/m2, can be described by the following equation:Q=α(Tfg−Te),  (2)where α can represent a heat-transfer coefficient in units W/m2-K. The temperature of the flue gas, Tfg, may not exceed an adiabatic flame temperature. This adiabatic flame temperature, for example, can be raised by preheating the combustion air and/or by using pure oxygen instead of air as a combustion gas. A reduction of the emitter operating temperature, Te, which can increase the convective heat transfer rate Q as suggested by Eq. (2), may not be desirable because it can also reduce the radiation output. Greater convective heat transfer may be achieved, e.g., by increasing the heat-transfer coefficient α. This parameter can be varied by varying fluid dynamical behavior and surface characteristics within the combustion chamber.
Gas-fired IR emitters which may be used in the paper and/or textile industry can be classified into general groups based on the material used to form them, e.g., fiber emitters, ceramic emitters and metal emitters. Such emitters can preferably have some or all of the following features: long service life, ignition reliability, high output level, short heat-up and/or cool-down time, and cross-directional moisture control.
A fiber emitter can include a fiber mesh formed of, for example, metal fibers that can be approximately 3 mm thick or ceramic fibers which may be up to approximately 25 mm thick. Such fibers can provide a temperature barrier in fiber emitters. A gas/air mixture can flow through almost the entire cross-sectional surface of the fiber mesh, which can cool the mesh and also lead to relatively low gas-flow velocities. The actual combustion process can occur at least partially in the outer layer of the fiber.
Fiber emitters may be of limited use for applications in the paper industry because of low flow velocities and unfavorable convective heat transfer performance (which may operate at, e.g., approximately 30-35% efficiency). Also, a flame in such an emitter can be easily disturbed by secondary air currents such as those which may be associated with an adjacent moving sheet or web. Increasing the thermal input power supplied to such an emitter (e.g., by increasing the gas/air flow rate) above a critical level can causes the flame to rise above the fiber surface and burn unevenly. Such uneven combustion can be referred to as a “blue flame mode” and can lead to a rapid decrease in the level of radiation output from the emitter.
Gas-fired infrared (“IR”) emitters which may be used in the paper and/or textile industry can be classified into general groups based on the material used to form them, e.g., fiber emitters, ceramic emitters and metal emitters. Such emitters can preferably have some or all of the following features: long service life, ignition reliability, high output level, short heat-up and/or cool-down time, and cross-directional moisture control.
A fiber emitter may be prone to damage caused by clogging of the fiber mesh. Gas flow may be blocked in certain portions of the mesh, which can cause an irreversible reduction of the radiating surface. Such degradation in a fiber emitter performance can be referred to as a “coating-color splash.”
As the flame burns in an outermost layer of a fiber mesh (where the outer layer may be, e.g., approximately 0.3 mm deep) and the mesh is relatively open, short heat-up and cool-down periods may be achieved. For example, the fibers may reach an operating temperature within about 3 seconds and may cool down at a similar rate. However, the advantages of such a short cool down time may be partly offset by the considerably longer cool-down times which may be present for connected housing components. In contrast to the fiber material, these components may not be cooled by the flow of combustion air after the emitter has been turned off, which can lead to a slower temperature decrease.
Fiber emitters may thus be preferable for applications where brief heat-up and cool-down periods take priority over robustness and performance. The disadvantages of fiber emitters described herein can be compensated for to some extent, e.g., by providing a secondary radiation element such as, for example, a screen that may partially shield the combustion chamber. Combustion gases or fumes can first contact the metal or ceramic fibers of the emitter and transfer a certain amount of heat. The fumes may then exit the fiber mesh and exchange further heat with the screen at a lower temperature. The heated screen may thus provide a secondary source of infrared radiation. Also, the screen can reflect some of the heat back toward the fiber, which may intensify combustion on the fiber surface. This intensification can also inhibit separation of the flame from the mesh surface at high temperatures. Such effects can be used to raise the output level of a fiber emitter, e.g., to approximately 44%. The use of a secondary radiation element may also reduce susceptibility of the flame to disturbances from air currents, and thereby reduce the occurrence of coating-color splashes. However, providing a secondary element can introduce additional mass that may be heated up and cooled down, which can reduce the advantages described herein for emitters having a low thermal inertia.
Ceramic emitters can include a perforated ceramic plate which may provide a temperature barrier. The number and size of the holes in the ceramic plate can be configured to provide a higher gas-flow velocity than may be present in fiber emitters. Thus, combustion processes associated with ceramic emitters may exhibit enhanced stability with respect to secondary air currents. The output level of such emitters may also be relatively low (e.g., approximately 44%). A screen may be generally used as a secondary radiation element with ceramic emitters, because heat reflected by the screen can be used to ignite the gas/air mixture as it exits the perforated ceramic plate. Although most characteristics of a ceramic emitter may be comparable to those of a fiber emitter, ceramic emitters can provide a higher power density because refractory ceramic plates can operate at higher temperatures than conventional mesh materials.
Metal emitters can provide an encapsulated combustion chamber which shields combustion and heat-transfer processes from outward influences such as, e.g., secondary air currents and coating-color splashes. This can be achieved by providing steel plate-formed elements into a screen which may be otherwise similar to screens used in other types of emitters. Metal emitters can provide improved convective heat transfer rates for a given volumetric flow rate of gases using an impinging flow. For example, a metal nozzle plate and a vacuum-formed ceramic structure may be provided instead of a perforated ceramic plate. The lower number of gas-flow orifices in such a configuration can produce increased gas-flow exit velocities.
A metal emitter may be configured using impingement plates (e.g., IR radiators) provided in proximity to nozzle orifices. Such configuration may provide improved heat transfer efficiency which may be as high as, e.g., approximately 52%. A vacuum-formed structure that may be used in metal emitters (instead of, e.g., the perforated ceramic plate used in ceramic emitters) can provide improved insulating properties and may be less susceptible to damage by a thermal and/or mechanical shock. A relatively weak, vacuum-formed ceramic insulation can be used in the metal emitters because the metal nozzle plate can provide or increase mechanical stability. However, metal emitters may have a large thermal inertia because they can contain a greater mass of material that is heated during use. Operating procedures may often be adjusted to reduce the negative effects of this larger thermal mass.
High temperature alloys may often be used in applications relating to energy conversion including, e.g., emitters, combustion engines, etc. Desirable properties of alloys in such applications can include, e.g., low weight and good mechanical stability. These properties can allow cost-effective and environmentally compatible operation of facilities such as, e.g., power plants and vehicles including cars, trucks and airplanes. Materials used in high temperature applications can be exposed to aggressive environments, and their performance, mechanical integrity and/or usable lifetime can be impaired by oxidation and/or corrosion. For example, a formation of a stable protective oxide scale on a surface of such materials can protect them at high temperatures. An aluminum oxide compound (alumina, e.g., α-Al2O3), can provide effective protective properties due to its high thermodynamic stability and relatively slow growth rate. Thus a mechanically stable layer of alumina may be desirable to protect materials used in high-temperature applications. Materials which can form alumina can include, e.g., aluminum alloys, intermetallics, superalloys, Fe—Cr—Al alloys and other ferritic high chromium alloys.
The rate of growth and quality of an oxide surface (e.g., alumina) can be a significant factor in determining service life of a high-temperature component. For example, oxide layers may spall (e.g., crack and peel off), and spalling may become more pronounced in thicker oxide layers. A reduction of oxide growth rates on a material under high-temperature service conditions may also reduce the extent of spalling and can increase service life. Growth of a higher quality oxide layer may also reduce the occurrence of spalling, and may further enhance the service life. A loss of oxide layers from spalling can also lead to an additional oxidation of underlying material, and may impair the mechanical integrity of the material.
Improving the emissivity of an metal emitter can enhance energy efficiency and energy conservation, and may lead to improved service lifetimes by limiting oxide formation and reducing the occurrence of spalling. One of the objects of the present invention is to provide materials and methods for making them which overcome some of the above-described deficiencies.